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Vehicle Efficiency Pathways

How modern passenger cars are removing themselves from the environmental debate

John Bucknell

GM Powertrain

219 April 08

Abstract

Modern passenger cars must respond to market demand and regulation forces, delivering superior air quality, utmost safety and ever-higher energy efficiency. This lecture will discuss efficiencies on both the supply and demand pathways for improving energy efficiency in the context of emissions and safety regulations. Well-to-wheel and pump-to-wheel efficiencies will also be covered in brief to highlight the efficiency of Electric Vehicles

319 April 08

Topics Transportation Efficiency

State of the industry

Supply-side Efficiency Powertrain Efficiency Driveline Efficiency Load-leveling

Demand-side efficiency Aero, rolling-resistance, inertia

Electric Vehicles & Fuel-Cell Vehicles Pump-to-wheels, well-to-wheels

419 April 08

Market Economics Cost of ownership

Market demand has illustrated that customers will purchase what they can afford. Technologies that increase cost of ownership have great difficulty penetrating the market.

Energy Costs

Dual impact of increasing environmentalism and increasing energy costs have raised the visibility of vehicle efficiency.

Low energy cost of petroleum products has been the primary factor that has driven the market into a near monoculture for it’s energy needs.

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Transportation Efficiency State of the industry

Economies of scale drive allow manufacturers to compete on cost. Any technology that cannot make a component at a minimum rate of one per minute requires additional sets of tooling, driving up investment and increasing the number of sales to break even.

Profit margins in the automotive industry are exceptionally small, as you’d expect with strong competition for a very large revenue stream.

State of the world has changed rapidly – developing new technologies that are sufficiently robust to be used by every consumer can take a decade or more. The industry is responding to the need for greater efficiency, vehicles on the market today are just the beginning.

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Regulation Tailpipe Emissions

Air quality has been driven by the EPA and the California Air Resources Board. Details on how emittants are formed and regulated follow.

Passenger safety

Customer awareness of impact performance on standardized tests has driven the industry to achieve a minimum “Four star” rating in any test. The degree that of likelihood of injury to achieve the best rating has decreased significantly over the last ten years. High strain-energy density materials, and large masses of them have driven up body structure mass by about double in the same time frame.

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Emission and Fuel Economy Test Cycles

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Engine Fuel Balance

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0

0.1

0.2

0.3

0.4

0.5

0 0.1 0.2 0.3 0.4 0.5

NOx Standard

HC S

tanda

rd

1983 Federal Tier 0

Federal Tier 1

History of Emission Control Standards

0

1

2

3

4

5

6

7

8

9

10

11

12

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

NOx Standard

HC S

tandar

d

Typical 1960 Vehicle (pre-control)

1971 California Std.1977 Federal Std.

NLEVLEV2

ULEV2

Emissions Standards 1960 to 2008

SULEV2

99.99%Reduction

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Exhaust Aftertreatment

Catalysts have the capability of modifying the reaction rates of chemical processes (typically increasing reaction rates) without being consumed while doing so.

The following chemical processes are of interest in automotive exhaust catalytic aftertreatment

• HC + O2 CO2 + H2O

• CO + O2 CO2

• NO N2 + O2

• These reactions proceed toward equilibrium at very slow rates at prevailing exhaust temperatures - catalysts increase their reaction rates to a degree that the exhaust aftertreatment becomes practical.

Conversion efficiency: (inlet concentration - outlet concentration)/inlet concentration

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SubstrateMat Can Washcoat Catalysts

Source: Corning (2001)

Essential Components of a Catalytic Converter

Substrate: a ceramic honeycomb-like structure with thousands of parallel channels for applications of washcoat and catalysts

Mat: Provides thermal insulation and protects against mechanical shock and chassis vibration

Can: A metal package encasing the catalyzed substrate and mat

Washcoat: a coating that increases the surface area of the substrate for catalysis

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Catalysts for Exhaust Aftertreatment

The active catalytic material is typically a blend of platinum, palladium, rhodium and nickel.

Small amounts of these materials are distributed on a alumina (Al2O3) washcoat, which is specially processed to have very high microscopic surface area. The high washcoat surface area helps to keep the catalytic material spread out to reduce the tendency to agglomerate and thus loose surface area.

Cerium oxide is often added to this mix to mechanically stabilize the alumina microstructure against thermal degradation.

Typically there are 0.5-2 grams of catalytic material per liter of overall catalyst volume, and the overall catalyst volume is about 50 ~ 80% of the engine displacement, depending on the application.

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Temperature Effects on Catalyst Capabilities

Catalyst efficiency at catalyst temperatures below 200oC is extremely low.

Catalyst efficiency rapidly increases as its temperature rises above 200oC and reaches its temperature plateau at about 400oC.

Light-off temperature: conversion efficiency reaches 50%

Current exhaust system design practice insures catalyst light-off within ~ 20 seconds without special aids. Catalyst heating devices in lowest emissions vehicles can achieve light-off in under 10 seconds.

Source: Heywood (1988)

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Catalyst Efficiency with Air/Fuel Ratio

Source: Heywood (1988)

Steady improvements in fuelling control, engine-out emissions and catalyst technology has made it possible to achieve 100% conversion rates of HC and NOx after catalyst light-off.

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Emissions SummaryFuel-burning engines create pollutants that are regulated - which are ever-more stringent. Emissions-control technology has evolved to the point where three-way catalysts are 100% efficient in converting HC, NOx and CO – only if the feedgas operates very close to stoichimetric air-fuel ratio.

Any lean-burning combustion process (Diesel or stratified charge) which improves fuel consumption also prevents catalytic NOx reduction by maintaining oxygen in the exhaust stream. Several technologies are emerging which consume fuel or reductant to purge Lean NOx Traps, at the cost of fuel consumption or added complexity.

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Supply-Side EfficiencyEnergy conversation pathway

Powertrain Efficiency (Stratified Charge/HCCI, Downsizing/Boosting)

Driveline Efficiency (Multi-speed Transmissions, CVTs) Load-leveling (stop-start, mild hybrid, series and parallel

hybrids)

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Energy Distribution in Passenger Car Engines

Source: SAE 2000-01-2902 (Ricardo)

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Modern Naturally Aspirated Brake Thermal Efficiency Map

0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0.05

0.1

0.15

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0.25

0.3

0.35

Fra

cti

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Maxim

um

To

rqu

e

Fraction Maximum Engine Speed

Fra

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herm

al E

fficie

ncy

2019 April 08

Compression vs Spark Ignition

Compression ignition achieves significantly higher compression ratios than spark ignition – raising thermal efficiency

Spark ignition engines control load by throttling, introducing parasitic losses at less than maximum load which reduces thermal efficiency

Smoke limits reduce power density of diesel engines to only about 80% of energy density of spark ignited of similar displacement. High operating pressures require heavy construction which further lowers power/weight ratio

Source: Heisler (1995)

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Powertrain Efficiency Pathways- Engine -

Prior three slides show that the maximum fraction of fuel energy that reaches the brake is 30-40% of the fuel input energy, which is the most that thermodynamics allows.

Spark ignition engines pay a loss to reduce load by throttling – which is effectively operating a vacuum pump. Several technologies seek to reduce or eliminate pumping work:

• Exhaust Gas Recirculation (EGR) – load reduction by diluting incoming combustion air

• Variable valve timing (including cam phasers and variable lift/duration systems) – load reduction by reducing trapping efficiency and adding residual (internal EGR)

• Stratified Charge with unthrottled operation – load control via fuel mass running lean

• Homogeneous Charge Compression Ignition (HCCI) – load control via fuel mass and residual preventing lean operation

• Downsizing/Boosting – Reduction in displacement of engine so use of lowest efficiencies is mostly avoided and then boosting to enhance available load

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Powertrain Efficiency Pathways- Driveline -

Knowing that an internal combustion engine is most efficient in a limited regime, the driveline can be optimized to enable engine operation the least amount of time away from that regime.

• Multi-speed Transmissions – 6, 7, 8 speeds with ratio ranges from 5.0-6.0 give powertrain controller best option of matching engine to current power demand

• Continuously Variable Transmissions – Same as multi-speed transmissions, but typically have high parasitic losses

• Load Leveling – Through use of onboard energy storage (electric or other), energy conversion can happen at most efficient point in map. Hybrids achieve this through several different strategies – parallel, series or dual-mode are most-often discussed. Micro-hybridization also appearing due to low cost of implementation.

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Load-LevelingEngine Stop Start (ESS)

Eliminates fuel consumed during deceleration and idle

Fuel On

Fuel On

Fuel Off

Fuel Off

Source: SAE 2001-01-0326 (GM)

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Load-Leveling

Source: SAE 2006-01-1502 (GM)

Mild Hybrid

Regenerative Braking, Load-Leveling and Idle Stop

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Load-LevelingStrong Hybrid

Electric-only operation, Regenerative Braking, Load-Leveling and Idle Stop

Parallel, Series and Two-Mode e-CVTs

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Demand-Side Efficiency Not a true ‘efficiency’, however losses that are not

minimized could be considered ‘in-efficient’

Major Components

Inertia Loads (Kinetic Energy)

Aerodynamics

Rolling Resistance

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Demand-Side Efficiency Inertia Loads

Vehicle mass requires proportional power to accelerate. Vehicle duty cycles with greater time spent accelerating will be more sensitive to vehicle mass.

Aerodynamics

Pressure drag: The loss due to the difference in pressure on the front face versus the rear face of the vehicle. The dynamic pressure (also called stagnation pressure) on the leading face is a measure of the kinetic energy of the displaced air.

Friction drag: Losses due to viscosity effects are also substantial. Boundary layer theory says that particles immediately next to a vehicle must be moving at vehicle speed as compared to at the free stream velocity. The shear force created by the relative velocity of the fluid is proportional to vehicle speed and ‘wetted’ surface area moving through the fluid.

The two speed-dependent components cause aerodynamic drag to increase primarily with the square of vehicle speed

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Demand-Side Efficiency Rolling-resistance

Driveline: Seals, bearings, gears, CV and Cardon (universal) joints Any component using a viscous fluid to reduce contact stress

for increased durability also suffers the losses of viscous shear forces regardless of the load.

Brakes Friction brakes work by rubbing two components together.

Unfortunately due to the balancing of pad retraction and response time, disc brakes will drag the pads against the rotors – a little or a lot depending on the design. Drum brakes by their nature have very little hydraulic volume and thus can retract far enough to not drag.

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Demand-Side Efficiency Rolling-resistance

Tires Part of the suspension isolating the vehicle from surface

irregularities. Tire is both a spring and a damper, with greater spring rate and lesser damping force with lesser sidewall height. Spring rate is proportional to inflation pressure. Greater isolation drives greater sidewall and lower pressure.

Inflation pressure is same as tire contact pressure. Contact area is proportional to mass supported by the tire. The greater the contact area, the more rubber has to deflect as it tracks across the surface. Increased tire diameter decreases the degree of deflection. Rubber is not perfectly elastic, so some energy is lost.

The force to roll a tire is therefore proportional to the normal force and the volume of rubber deflected per second which is proportional to rotational velocity.

3019 April 08

Measuring Vehicle Efficiency EPA and real world fuel economy (Efficiency) is impacted by the

vehicle’s drag force. Drag is determined by taking a vehicle to 70 mph and then shifting into neutral and measuring speed versus time and thus deceleration rate. Knowing the mass of the vehicle, a drag force versus vehicle speed can be derived. This drag force data is fitted to a 2nd-order polynomial whose coefficients are published by the EPA – called the ABC coefficients.

The chassis dyno where emissions and fuel economy data is taken has Rollers instead of pavement, with vehicle strapped down Only drive-wheels turning No aerodynamic loading

The A,B,C coefficients determine the load which the dyno program must match over the course of the test cycle

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Vehicle Drag Force Example

A = 28.73 lbB = 0.7338 lb/mph

C = 0.01084 lb/mph^2

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Evidence of Vehicle Efficiency EPA data shows that there is no magic. Following slides show

every vehicle for sale in 2008 Model Year in the US.

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Inertial Loads

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Aerodynamics

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Rolling Resistance

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Downsizing

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Typical Mid-Size Vehicle Energy Distribution Idle 0.90

Engine Losses 5.97

Accessories 0.17

Driveline Losses 0.25

Aero 0.21

Rolling 0.34

Kinetic

Braking 0.45

Engine D/L 1.00 1.258.29 units

Urban Federal Test Procedure (FTP)

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FTP City – Mid-Size Sedan Simulation

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Simulation – Level of Hybridization

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Simulation – Hybrids with Downsizing

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Simulation – Advanced Powertrain Tech

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Electric Vehicles & Fuel-Cell Vehicles

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Well-to-Wheels and Tank-to-Wheels

Any true discussion of energy diversity and it’s impact on GHG must discuss the source of energy (ie the Well)

Electric Vehicles will receive the bulk of their energy from coal-fired generation for foreseeable future

Coal-fired electrical generation was 35% thermally efficient in 2005 (EPA)

Line-losses and battery/e-motor efficiency aren’t 0%

Therefore from a GHG perspective -TNSTAAFL

USA Energy Consumption (%) 1960

1970

1980

1990

2000

Oil 44.1 43.5 43.6 39.8 38.5

Natural Gas 27.5 32.1 26.0 22.9 23.7

Coal 21.8 18.1 19.6 22.8 22.7

Nuclear Energy .002 0.35 3.5 7.3 8.1

Hydro-,Geothermal,Solar, Wind,etc

6.6 6.0 7.2 7.4 6.9USA Electricity Generation (%)

1990

2000

Coal 52.6 51.8

Petroleum 4.1 2.9

Natural Gas 12.5 15.7

Nuclear 19.1 19.9

Hydroelectric 9.7 7.2

Geothermal 0.5 0.4

Wood 1.0 1.0

Waste 0.4 0.6

Other Waste 0.076

.09

Wind 0.099

0.129

Solar 0.020

0.021

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Transportation Effects on GHG - Future

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By 2020,

1.1 billion vehicles (an increase of 300 million) will circle the earth 125 times.

Energy diversity is required in the future. Reducing dependence on petroleum is imperative.

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“At GM, we believe tomorrow’s automobiles must be flexible

enough to accommodate many different energy sources.”

“At GM, we believe tomorrow’s automobiles must be flexible

enough to accommodate many different energy sources.”

- Rick WagonerChairman and CEOGeneral Motors CorporationLA Auto Show 11/29/2006

“ And a key part of that flexibility will be enabled by the development

of electrically driven cars.”

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Hybrid, Electric & Fuel Cell Vehicles Introduction & Background – More definitions

Vehicle TypeElectric Power

Onboard Electric Storage

Grid ConnectedRecharging?

Electric- only Driving

Mild HEV low low no no

Full HEV med low no limited

PHEV med med yes limited

E-REV high high yes Full

Ele

ctr

ificati

on

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1st & 2nd Generation Biofuels

Transportation Challenge – Energy Diversity - Source Blending via Electrification

Energy Resource Energy Carrier Propulsion System Conversion

BiomassBiomass

CoalCoal

Natural GasNatural Gas

Renewables (Solar, Wind, Hydro)Renewables

(Solar, Wind, Hydro)

NuclearNuclear

Energy Resource Conversion Energy CarrierPropulsion System

Conventional ICE:Gasoline/Diesel

Conventional ICE:Gasoline/Diesel

Mild and Full Hybrids

Mild and Full Hybrids

Plug-In Hybrid

Plug-In Hybrid

Battery Electric Vehicle

Battery Electric Vehicle

FC Electric VehicleFC Electric Vehicle

Thermochemical

Water-Splitting

Oil(Conventional)Oil(Conventional)

OilOil (Non-Conventiona

l)

(Non-Conventiona

l)

Petroleum Fuels

SyngasCO, H2

FischerTropsc

hCCGT

ShiftReactio

n

LiquidFuels

Hydrogen

Electrolysis

CO2 Sequestration

Extended Range EV

Batt

ery

Energ

y S

tora

ge

Batt

ery

Energ

y S

tora

ge

Electricity

More

Ele

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fica

tion

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Transportation Challenge – Energy Diversity - Source Blending via Electrification

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Advanced Technology and Sustainability… GM Technology StrategyAdvanced Technology and Sustainability… GM Technology Strategy

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Chevrolet VOLT Concepts Illustrate E-REV and FC Commonality

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Chevrolet VOLT E-REV Concept

• Global Compact Vehicle Based

• Electric Drive Motor• 120 kW peak power

• 320 Nm peak torque

• Li Ion Battery Pack• 136 kW peak power

• 16 kWh energy content

• Home plug in charging

• Generator• 53 kW peak power

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E-Flex Fuel Cell Variant

• Global Compact Vehicle Based

• Electric Drive Motor• 120 kW peak power

• 320 Nm peak torque

• Fuel Cell Propulsion System

• Smaller Li Ion battery pack

• Hydrogen storage

5519 April 08

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